The invention relates to a method for the processing of a received signal of an optoelectronic sensor, in particular of a light grid, as well as an apparatus for the carrying out of such a method.
In automation and security engineering, the use of light grids is known in which a plurality of parallel light beams are periodically transmitted to be able to monitor, for example, a planar surface for the intrusion of an object or of a body part. The received signal associated with each light beam is ultimately compared with a threshold value in a comparator stage in order to trigger an object detection signal or a switch-off signal on the threshold being fallen below or being exceeded. The signal processing chain at the reception side typically contains—for each reception channel—a photodiode as a light receiver, a transimpedance amplifier with a band pass filter property, a difference amplifier stage with a differential current output and the comparator stage. In addition, a control circuit can also be provided for background light suppression.
With respect to the plurality of reception channels, a realization of the evaluation electronics is desired which is as cost-favorable as possible. It is in particular desirable to combine the analog received signal processing for each reception channel at least partly into one integrated circuit, with these circuits being switched in parallel to form an analog bus. For the further reduction of the manufacturing effort, a single, common comparator stage can be provided for all reception channels of this analog bus. All received signals, i.e. all output signals of the respective difference amplifier stages, are then supplied to the comparator stage via the analog bus such that the comparator stage can check for every single reception channel whether a threshold has been exceeded or fallen below.
It is problematic that the integrated circuits associated with the individual reception channels have comparatively large tolerances due to production variance such that production-induced fluctuations which are very high in comparison with one another occur at the output of the respective difference amplifier stages. These fluctuations can result in the comparator determining an exceeding or falling below of the predetermined threshold value with respect to certain reception channels without a light reception having taken place at the associated reception channel. This can result, for example, in no switch-off signal being triggered despite an interruption of a light beam.
This danger could admittedly be eliminated in that a high pass filter is interposed before the comparator. However, it proves to be difficult to find a cut-off frequency for this high pass filter which satisfies the demands of the total system. Too low a cut-off frequency can result in the variation of the respective offset likewise being passed through to the comparator stage on a switching over between the different reception channels and thereby bringing about a switching of the comparator. Too high a cut-off frequency of the high pass filter necessarily lies in the transmission range of the band pass filter of the reception channel or of the integrated circuit. Its band pass filter limits are also necessarily subject to large fluctuations. The interaction with the interposed high pass filter can therefore result in a band pass filter of a higher order with a non-defined transmission function so that the overshoots in the time range resulting from this can again trigger an accidental switching of the comparator stage.